OK! EXCLUSIVE: Cameron Fights For Justin!

From the moment Cameron Diaz reunited with ex-love Justin Timberlake for January’s Hope For Haiti Now telethon, their bitter 2007 breakup seemed long ago and far away — and she got that old feeling all over again. Sure, Justin’s deeply involved with Jessica Biel. But as a Cameron insider tells OK!, “Cam had forgotten what great chemistry she has with Justin.”

So great that it was Cameron, 37, who fought fiercely for Columbia Pictures to cast acting novice Justin, 29, as her leading man in their sexy comedy Bad Teacher, now shooting in L.A. — passing over an A-list lineup including Ryan Reynolds, Paul Rudd and Bradley Cooper.

“Cameron kept telling them she wanted Justin,” reveals the source. “She was bummed because the studio said no — they didn’t think Justin would be good on the big screen.”

Still, the perseverant blonde would not be denied. “Cameron melted down and really pushed it,” the insider tells OK!, and when she showed clips from Justin’s comedy The Social Network, about the birth of Facebook, “that sold them.”

But what really made Cameron battle so hard for her ex? Was it merely his deft comic timing — or that she knew the Bad Teacher script called for steamy love scenes?

The film tells the story of a school teacher (Cameron) pursuing a spoken-for colleague (Justin).

“Cameron’s such a bad girl: she made them rewrite the love scenes so that they’re even juicier — and longer,” the source reveals. “She doesn’t want to stir up trouble. But you know, she’s been cheated on — Justin cheated on her. She may not hold to the letter of the law when it comes to Jessica.”

Now, after three years with Justin, it’s Jessica’s turn to get upset. While initially “freaking out” over Justin and Cameron’s lusty screen pairing, says another source, Jessica has since relaxed her attitude.

“When Justin made it clear he was doing the movie with or without Jessica’s support, Jess swallowed her pride and did the unthinkable — she pilfered Cam’s cell number from Justin’s phone and called her to bury the hatchet,” the source tells OK!. “Jessica said she was thrilled Justin was working with someone of Cameron’s caliber and thanked Cam for going to bat for him.”